The present invention relates to a coating used to protect surfaces from Ultra Violet degradation. Specifically, the present invention relates to a submicronized zinc oxide system which when used on various substrates, protects such from degradation from weathering.
It has been known that zinc oxide is useful as a mold growth inhibitor in paints, Ultra Violet (UV) absorber in plastics, ceramics, floor tile, glass, feed additive, dietary supplement, cosmetics, and so forth. The particle sizes of such zinc oxide (ZnO), as measured by their diameter however, has been about 0.11-0.9 microns. Disadvantageously, these larger particle sizes will not provide the features of the present invention, i.e., a clear or semi-transparent coating that protects the coating itself and the substrate from UV degradation.
Previously, the use of smaller particle sizes such as taught by the present invention would have been detrimental to those oil based systems such as Alkyds and latex vehicles because ZnO is reactive with acids and chlorine groups found in these systems. This results generally in crosslinking and/or destabilization of the composition as demonstrated by increased viscosity. The final impact is that of an unstable system. Thus, the idea of using ultra-fine or submicronized zinc in a latex system as taught by the present invention would have been remote.
More recently, Sumitomo Cement Co., Japan has developed a transparent zinc oxide that has been deprived of pigment property by a unique process of turning zinc oxide into ultra fine particles (UFP), in the range of 0.005-0.015 microns. The article states that this UFP zinc oxide is monodispersed and can be applied in such forms for coated fabrics, printing inks and paints. A more complete description and discussion of this UFP zinc oxide is described in Antibacterial, Deodorizing, and UV Absorbing Materials Obtained with Zinc Oxide (ZnO) Coated Fabrics by Mitumasa Saito, Sumitomo Cement Co, Japan, published by Journal of Coated Fabrics, Volume 23xe2x80x94October 1993.
The use of zinc oxide as a UV protector is seen in Published Japanese Application JP 7-278-465. This application teaches the use of zinc oxide with an average particle diameter of about 0.01 microns or less that is mixed with a polyacrylic acid ester resin having alkoxysilane groups as its side chains. This polyacrylic acid ester is a sunproofing paint composition used on various substrates such as cellulose-based wood, cloth or paper, protein based leather or cloth, or synthetic resin based leather, cloth or paper.
European Patent Application number 0 599 492 A1 teaches the use of titanium dioxide with an average particle size of 0.005 to 0.15 micrometer and having a coating of zinc oxide or hydrated zinc oxide in an amount of at least 100 wt-% with respect to TiO2. The invention is used in products such as sunscreens in which it protects against UVA and UVB light.
JP 6-297630(A) describes a transparent, UV absorbent protective film, characterized by the fact that at least a UV absorbent layer and a protective layer containing dispersed microparticles of titanium dioxide and zinc oxide. Both ingredients have a particle diameter of 0.05 microns or less and are successively formed, either directly or via an interlying anchoring treatment layer, on one side of a base film with at least an adhesive layer formed on the other side.
None of the teachings above fulfill the need provided by the present invention which is a coating composition that utilizes submicronized zinc oxide having a particle size of about 0.01-0.08 microns in a polymer latex to produce a composition that is useful as a protectant against UV degradation and weathering for wood surfaces.
The present invention relates to the use of submicronized zinc oxide in a polymer latex. The latex binder is made by the polymerization of at least one ethylenically unsaturated monomer or the latex from the post emulsification of condensation polymers such as esters of acrylic acid and methacrylic acid, vinyl esters, dienes and the like, with an effective amount of viscosity control agent as a protectant against UV degradation and weathering on substrates such as, for example, wood, by applying a coating composition containing the above ingredients to the substrate.
Accordingly, the present invention is a coating composition comprising an exterior durable latex polymer having a solids content of about 40-65 wt-%, an effective amount of submicronized zinc oxide, water and an effective amount of viscosity control agent.
Submicronized zinc oxide is generally characterized as having an average particle size of about 0.01-0.08 microns. By using these very small particles, greatly improved visible light transmission is possible, with also greatly improved UV absorption due to the increased surface area and number of particles. Using 0.02 micron versus 0.10 micron size zinc oxide increases the surface area five fold and the number of particles 125 fold. Preferably, the submicronized zinc oxide has a particle size of about less than 0.06 microns and the viscosity control agent is a cellulosic or associative type thickener commonly used in water base coating formulations.
The present invention can also include optional ingredients such as coalescent agents, wetting aids, adhesion promoters and pigments. Also included within the scope of this invention is an article whose surface is coated with a cured layer of the compositions of the present invention. Further, the present invention may also find use as a fungicidal protective coating on food preparation floors, as a clear protective coating for all wood surfaces such as home sidings and furnishings, on vehicles, lawn equipment, farm equipment, any processing equipment, on shingles or any other roofing materials and any other substrate that requires protection from algae, fungi/bacteria growth and/or UV protection.